Upon activation by antigen, B cells undergo antibody class (isotype) switching, changing from expression of IgM to expression of IgG, IgA or IgE, while maintaining specificity for the same antigen. Since the isotype determines the effector function of the antibody, class switching allows the humoral immune response to adaptively respond to different infectious organisms. Class switching occurs by a DNA recombination event between switch (S) region sequences located upstream of each heavy chain constant (CH) region gene. This process has mechanistic similarities to somatic hypermutation of Ig variable region genes. It has recently become clear that activation-induced cytidine deaminase (AID) initiates class switch recombination (CSR) by deamination of dC residues within S regions, creating dU residues. The resulting dU residues are excised by uracil DNA glycosylase (UNG) leaving a basic residues, and mice and humans without UNG have greatly reduced abilities to undergo CSR. However, in order to initiate CSR, the abasic site must be converted to a single strand DNA break. This has been hypothesized to be due to AP endonuclease (APE) but which of the two APEs might be involved are unknown. Another possibility is supported by data from this group, suggesting that the endonuclease ERCC1/XPF might also have this role. One of the goals of this grant is to determine how the initiating DNA breaks are introduced. A related goal is to determine the mechanisms for introduction of mutations into S regions during the DNA repair processes accompanying CSR. There are 4 specific aims directed towards these goals. Aim 1: to determine if components of the nucleotide excision repair (NER) pathway, in addition to ERCC1/XPF, are involved in CSR and to determine their role. Aim 2: to determine if the base excision repair (BER) enzymes APE1 or APE2 create the initiating DNA breaks in the Su segments. Aim 3: to determine if the BER and NER pathways are redundant for creation of the initiating DNA breaks and if there is a competition between these pathways. We will also explore if the competition is altered by activation of B cells in the presence of different induction conditions. In Aim 4 we will explore how the mutations are introduced into Ig S regions by determining if the translesion DNA polymerase iota is involved in the error-prone repair of S regions and if this involvement is regulated by cytokines.